Lithium ion secondary batteries, nickel hydrogen batteries, and other secondary batteries have increased in importance as power supplies installed in vehicles that use electricity as a drive source or power supplies used in electrical products such as personal computers and portable terminals, for example. A lithium ion secondary battery in particular is lightweight and exhibits a high energy density, and may therefore be used favorably as a high output power supply for installation in a vehicle.
In a typical configuration for a lithium ion secondary battery of this type, a wound electrode assembly formed in a flattened shape and obtained by winding and pressing a positive electrode sheet and a negative electrode sheet between which a separator sheet is interposed is housed in an angular battery case (typically a flattened box-shaped case) together with a non-aqueous electrolyte.
The non-aqueous electrolyte is a medium for conducting a carrier (typically cations, for example lithium ions) during charging and discharging of the secondary battery, and when an amount of non-aqueous electrolyte injected into the battery case is inappropriate, various battery characteristics cannot be exhibited sufficiently. When the amount of non-aqueous electrolyte is smaller than an appropriate amount, the electrolyte may be depleted (a so-called liquid shortage may occur) following repeated charging and discharging, leading to an increase in a battery resistance and a corresponding reduction in output. When the amount of non-aqueous electrolyte is larger than the appropriate amount, on the other hand, cations may flow out into surplus non-aqueous electrolyte that cannot be contained within the wound electrode assembly and therefore exists on the outside of the electrode assembly, leading to a reduction in output. These problems tend to occur particularly easily during use (charging and discharging) at a high rate (a high output of 5 C or more, for example). Patent Literature 1 to 5 may be cited as technical documents relating to an amount of electrolyte to be injected into a secondary battery. Patent Literature 1, for example, describes a technique of attempting to determine the amount of electrolyte to be injected from an opposing surface area of positive and negative electrodes.